A method of making high frequency quartz plates is disclosed in U.S. Pat. No. 4,274,907 issued June 23, 1981 to Vig et al. The method disclosed involves lapping the quartz plate with a fine abrasive, then etching the quartz plate with a chemical polishing etching solution, then polishing the quartz plate with a chemomechanical polishing process that produces a polished and undamaged surface, then etching the quartz plate with a chemical polishing etching solution. Such a method can produce very high frequency quartz plates, that is, fundamental mode frequencies in excess of 100 MHz. However, SC-cut plates that are greater than 100 MHz in frequency are less than 18 .mu.m in thickness. Such thin plates are very difficult to make into conventional resonators by conventional techniques. The primary reasons for the difficulty are the effects of mounting stresses, bonding stresses, and the difficulty of adjusting the frequency. Another method of making high frequency quartz plate is described in U.S. Pat. No. 3,694,677 issued Sept. 26, 1972 to Guttwein et al. In the Guttwein et al method, the thickness of a portion of the center of a conventional crystal plate is reduced by, for example, ion etching. The crystal plate is then mounted and sealed by conventional techniques. The typical embodiment of this invention is a circular plate of 8 mm diameter, the thickness of the center 5 mm diameter portion of which is reduced from 55.3 microns to 16.6 microns.
The Guttwein et al method has successfully produced high frequency resonators. The drawback of the Guttwein et al method is that, when it is implemented via ion etching, the process is very slow and the resulting resonators are prohibitive in cost. For example, an oscillator made with an ion-etched high frequency resonator costs about $8,000. Until recently, the method could not be properly implemented via chemical etching. That is, the quality of the etched surface might have reflected the preferential erosion by the etchant due to impurities or strains in the crystal lattice, or due to the inherent anisotropy of quartz. Moreover, the resultant surface might also have etch pits and etch channels that degrade the resonator Q and produce detrimental nonlinearities. Recently, some limited success has been achieved via the combination of the Guttwein et al method and the Vig et al chemical polishing etching method. The limitations of the use of the combined methods are due primarily to the etch channels and etch pits that are produced by the chemical polishing etching method. These channels and pits are due to defects that are present in all currently available quartz materials. The etch channel density of typical commercial quartz is on the order of 10 or 100 per cm.sup.2 for swept quartz, and is signficantly higher for unswept quartz. Therefore, since a conventional resonator plate's diameter ranges from 5 mm to 15 mm, the yield for making conventional resonator plates that are free of etch channels is extremely low, even when the much more expensive swept quartz is used. The etch pits and etch channels degrade the Q of the resonators. A large number of etch pits and etch channels will degrade the Q of the resonator to an extent that makes the resonator unusable. Even a small number of etch pits and etch channels degrades the resonator Q, and produces various nonlinear effects which significantly limit the usefulness of the resonator.